Phase-shifting element and particle beam device having a phase-shifting element

ABSTRACT

A phase-shifting element for shifting a phase of at least a portion of a particle beam is described, as well as a particle beam device having a phase-shifting element of this type. In the phase-shifting element and the particle beam device having a phase-shifting element, components shadowing the particle beam are avoided, so that proper information content is achieved and in which the phase contrast is essentially spatial frequency-independent. The phase-shifting element may have at least one means for generating a non-homogeneous or anisotropic potential. The particle beam device according to the system described herein may be provided with the phase-shifting element.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/931,046 filed Jan. 20, 2011 now U.S. Pat. No. 8,173,963 which is adivisional of U.S. application Ser. No. 12/070,055 filed Feb. 14, 2008now U.S. Pat. No. 7,902,506 (issued), which are hereby incorporated byreference.

TECHNICAL FIELD

This application relates to a phase-shifting element for shifting aphase of at least a portion of a particle beam, as well as a particlebeam device having a phase-shifting element of this type.

BACKGROUND OF THE INVENTION

The study of biological samples with the aid of a phase contrast haslong been known in light microscopy. Using a so-called phase ring, whichis situated in the back focal plane of a light microscope's objective,the radiation coming from a sample to be studied, which containsdiffracted and undiffracted components, becomes phase-shifted by 90°(i.e., by π/2) in the zeroth order of diffraction. The interference ofthe higher orders of diffraction with the phase-shifted beam delivers acontrast-rich image (image having a phase contrast).

The use of a method of this type in electron microscopy, in particularin transmission electron microscopy, has for long not been possible.However, even in that case the use of a phase contrast study isadvantageous because biological samples are basically very thin and havemostly very light-weight atoms, so that they are almost transparent forincident electrons/electron waves. Samples of this type are known asweak-phase objects. Instead of modifying the amplitude of an incidentelectron wave, weak-phase objects cause only a phase shift of theincident electron wave. To obtain a better phase contrast, an additional90° (i.e., π/2) phase shift between the scattered or diffracted and theunscattered or undiffracted electrons is needed.

Some proposals have already been made to achieve a phase contrast intransmission electron microscopy.

On the one hand, it has been proposed to achieve the phase contrast in atransmission electron microscope (hereinafter referred to as TEM), whichhas an objective lens, by defocusing the objective lens and thespherical aberration of the objective lens. It is, however,disadvantageous here that only a spatial frequency-dependent phase shiftis achieved via this procedure, so that the achieved phase contrast isalso spatial frequency-dependent. Samples to be studied having very lowspatial frequencies have, therefore, a very low contrast. Thisrelationship may be described in greater detail using the phase contrasttransfer function (PCTF or CTF for short) (see, for example, Optimizingphase contrast in transmission electron microscopy with an electrostatic(Boersch) phase plate, E. Majorovits et al, Ultramicroscopy 107 (2007)213-226).

On the other hand, it has been proposed to use a so-called Zernike phaseplate which provides a uniform phase shift of scattered or unscatteredelectrons for all spatial frequencies. A phase plate of this type ismade of a thin carbon film, for example, which is provided with a smallhole. This carbon film is situated in the back focal plane of anobjective lens of a TEM. Unscattered electrons pass through the smallhole of the carbon film, while scattered electrons hit the carbon filmitself and may suffer a phase shift of 90° (π/2) due to a homogeneousisotropic potential (internal potential, Coulomb potential) of thecarbon film. It has been found, however, that the carbon film is rapidlycontaminated. In particular the small hole is easily clogged.Furthermore, it is disadvantageous that the carbon film is rapidlycharged.

Furthermore, a so-called Hilbert phase plate which, like the Zernikephase plate, may also be made of a thin carbon film and causes the phaseshift of a half-space, is also used for phase shifting. However, theHilbert phase plate has the same disadvantages as the Zernike phaseplate.

An alternative form of the phase shift in a TEM is achieved by using aBoersch phase plate in which an electron beam is exposed to anelectrostatic potential. A design of such a Boersch phase plate isknown, for example, from EP 782 170 A2. It has a ring electrode and aholding device. The ring electrode is made of an annular plate having acentral opening and an outer edge. The holding device is formed by twostraight supports which are situated opposite each other. In this knownBoersch phase plate, the electrons unscattered or undiffracted on asample pass through the central opening, while the scattered ordiffracted electrons pass outside the annular plate. An electrostaticfield which causes a phase shift of the electrons passing through thecentral opening is formed in the central opening. However, the knownBoersch phase plate has the disadvantage that, in particular due to thesupport, partial beams are shadowed and prevented from contributing tothe interference, so that information is lost.

Another Boersch phase plate is known from WO 03/068399 A2. This documentrelates to a phase plate having a ring electrode which is designed as aplate having an opening and a peripheral outer edge. Furthermore, aholding device including supports is provided. The supports are appliedto the outer edge of the ring electrode and are used for positioning thering electrode in a TEM. This known phase plate is characterized in thatthe holding device does not have a central symmetric design and nofurther support is situated on the outer edge of the plate opposite toeach support with respect to the center of the opening. The known phaseplate is situated in the back focal plane of an objective of a TEM.Although, due to the supports of the phase plate, a partial area of thescattered electrons is shadowed in the back focal plane of theobjective, due to the non-centrally symmetrical arrangement, thescattered electrodes remain unaffected in a further partial area whichis centrally symmetrical to the shadowed partial area. Due to thesymmetry relationships, this phase plate allows the image informationinitially lost due to the shadowing by the supports to be reconstructed.The disadvantage of this known phase plate is, however, also that apartial area of the scattered electrodes is shadowed and therefore partof the image information must be reconstructed.

DE 102 00 645 A1 describes an electron microscope which also has aphase-shifting element. This phase-shifting element is ring-shaped andhas a central opening, and it is therefore able to be secured at itsouter periphery, so that no cantilever or almost-cantilever structuresare needed.

Accordingly, it would be desirable to provide a phase-shifting elementand a particle beam device having a phase-shifting element, in whichcomponents shadowing the particle beam are avoided, so that properinformation content is achieved and in which the phase contrast isessentially spatial frequency-independent at zero defocus (Gauss focus).

SUMMARY OF THE INVENTION

A phase-shifting element according to a system described herein isprovided for shifting the phase of at least a portion of a particlebeam. It may have at least one means for generating a non-homogeneouspotential which affects the particle beam. Alternatively oradditionally, at least one means for generating an anisotropic potentialmay be provided.

It is provided, in particular, that the phase-shifting element accordingto the system described herein may be placed in an electron beam device,for example in a transmission electron microscope. The phase shift maybe freely settable by selecting an appropriate non-homogeneous and/oranisotropic potential. For a good image contrast, a phase shift of 90°(π/2) is useful. Other phase shifts are, however, possible also in thecase of the system described herein. Accordingly, the system describedherein provides that the particles unscattered by an object suffer aphase shift relative to the particles scattered by the object. In otherwords, there is a relative phase shift between the particle beam ofzeroth order of diffraction and the diffracted particle beams of ahigher order of diffraction.

The phase-shifting element according to the system described herein hasthe advantage that interfering holding devices are no longer needed.Thus, contrary to the related art, in the case of the system describedherein no portion of a particle beam is suppressed due to shadowing. Avery good object signal and therefore good image information is thusobtained. In addition, the use of a non-homogeneous potential ensuresthat the phase contrast obtained is spatial frequency-independent atzero defocus (Gauss focus).

In a first exemplary embodiment of the phase-shifting element accordingto the system described herein, the means for generating anon-homogeneous potential is designed for generating a non-homogeneouselectrical potential. This may be easily accomplished. Furthermore, itallows a spatial frequency-independent phase contrast to be achieved inparticular. Alternatively, it is provided that the means for generatinga non-homogeneous potential is provided for generating a non-homogeneousmagnetic potential. The use of at least one permanent magnet, forexample, is provided for this purpose. The above also applies for acorresponding anisotropic potential.

It is furthermore provided that the phase-shifting element has at leastone opening which may be designed as a through opening through thematerial of the phase-shifting element. It should be expressly pointedout, however, that the opening does not need to be designed as a throughopening. For example, embodiments of the system described herein areprovided where the opening is not designed as a through opening. Forexample, openings are provided which are closed by a thin foil. Theopening may be designed not to be rotationally symmetric in theperpendicular direction to a plane of the phase-shifting element togenerate a good and highly localized non-homogeneous and/or anisotropicpotential.

In another embodiment of the system described herein it is provided thatthe phase-shifting element has precisely one opening. The scattered orunscattered particle beams (for example, electron beams in a TEM) passthrough the above-mentioned at least one opening or the precisely oneopening, the unscattered particle beam (i.e., the particle beam ofzeroth order of diffraction) suffering a relative phase shift withrespect to the particle beams of higher order of diffraction. Theinterference of the particle beams of higher order of diffraction withthe relatively phase-shifted particle beam provides a sufficiently good,contrast-rich image.

In particular it may be provided that the phase-shifting element has afirst element dimension along a first axis and a second elementdimension along a second axis, the first axis being non-parallel to thesecond axis. The opening of the phase-shifting element runs in thisexemplary embodiment both along the first axis and along the secondaxis.

Furthermore, one embodiment of the system described herein ischaracterized in that the opening has a first opening dimension alongthe first axis and a second opening dimension along the second axis, thefirst opening dimension being greater than the second opening dimension.In particular, the first axis and the second axis define a plane (forexample, the surface of the phase-shifting element) on which the openingis situated. The opening dimensions should be selected in such a waythat a diffraction pattern that is to be imaged is able to pass throughthe opening. In a particular embodiment, the first opening dimension maybe in a range of 20 μm to 200 μm, possibly in the range between 40 μmand 180 μm, and possibly in the range between 50 μm and 150 μm. Thesecond opening dimension may be in the range between 1 μm and 20 μm andpossibly between 2 μm and 5 μm.

The first axis may be perpendicular to the second axis. Thephase-shifting element may also have a third axis, which is not parallelto the first axis and not parallel to the second axis, but possiblyperpendicular to both the first and second axes. The opening then has athird opening dimension which extends along the third axis, the thirdopening dimension basically corresponding to the height of thephase-shifting element. It is basically freely selectable and may amountto between a few μm and a few mm.

It is provided in particular that the opening has a slot-shaped design.This is to be understood as a rectangular design in particular. Itshould be expressly pointed out that the slot-shaped design is notlimited to the shape of a rectangle, but it may have any conceivableshape to achieve a slot-shaped design. In a second embodiment it isalternatively or additionally provided that the opening has across-shaped design.

In a further embodiment of the phase-shifting element according to thesystem described herein, the phase-shifting element has a multilayerdesign. In this case it may be provided that the phase-shifting elementhas at least one electrode. However, other embodiments are also providedin which more than one electrode is situated on the phase-shiftingelement. In an exemplary embodiment, the electrode is made of gold.Furthermore, it is connected to a control unit via at least oneelectrical conductor to generate a non-homogeneous and/or anisotropicpotential.

In an embodiment of the system described herein, it is provided that theelectrode is segmented in such a way that it is made up of individualelectrode segments. The electrode segments are possibly individuallycontrollable.

Additionally or alternatively to segmentation of a single electrode, itis provided in another embodiment of the system described herein that aseparate first electrode and a separate second electrode are provided,which are also possibly individually controllable.

Furthermore it may be provided that the electrode has a first side and asecond side. A first insulator is situated on the first side of theelectrode. In addition, it may be provided that a second insulator issituated on the second side of the electrode. The first insulator and/orthe second insulator is/are possibly made of Al₂O₃. In a furtherembodiment it is provided that a first shield unit is situated on thefirst insulator. In addition, a second shield unit is situated on thesecond insulator. The first shield unit and/or the second shield unitis/are possibly made of gold.

The above-described embodiment(s) is/are easy to implement (for example,using lithography and/or sputtering). Furthermore, they are easilycontrollable from the outside. It is possible in particular to control anon-homogeneous or an anisotropic potential (for example, anon-homogeneous electric potential) in such a way that a desired phaseshift is obtained.

In another embodiment of the system described herein, the phase-shiftingelement has a drift unit. For example, it is provided that the openingof the phase-shifting element is designed as a drift unit or a driftunit is situated in the opening. The design and function of drift unitshave long been known, so that they will not be described in detail here.

In a further embodiment of the system described herein it is providedthat the above-described phase-shifting element additionally has atleast one section having an internal potential. It is thus thecombination of the phase-shifting element according to the systemdescribed herein with the known related art.

The system described herein also relates to a system having at least onefirst phase-shifting element and at least one second phase-shiftingelement, both the first phase-shifting element and the secondphase-shifting element having at least one of the above-mentionedfeatures or feature combinations. In particular, it may be provided thatthe system has an optical axis and the first phase-shifting element andthe second phase-shifting element are situated along the optical axis.In this case the second phase-shifting element is situated downstreamfrom the first phase-shifting element.

The system described herein is of advantage in particular when across-shaped design of the opening of the phase-shifting element isused. This may be implemented through the system described herein byalso using two phase-shifting elements connected in series and having anopening (for example, a rectangular slot) oriented in a preferreddirection. The phase-shifting elements are situated in series in such away that the preferred directions of the phase shifts are perpendicularto each other. A particle beam thus practically passes through across-shaped opening in two passes.

The system described herein also relates to a particle beam device whichis, provided with at least one objective which has a back focal plane.Furthermore, at least one diffraction lens for enlarged imaging of theback focal plane of the objective in an intermediate diffraction patternplane is situated on the particle beam device. At least one seconddiffraction lens is also provided which is situated in the intermediatediffraction pattern plane. “In the intermediate diffraction patternplane” (here and hereinafter) is to be understood, on the one hand, asexactly in the intermediate diffraction pattern plane and, on the otherhand, as near the intermediate diffraction pattern plane. The particlebeam device according to the system described herein is furthermoreprovided with at least one multipole element and at least onephase-shifting element, which has at least one of the above-mentionedfeatures or feature combinations and is possibly situated in theintermediate diffraction pattern plane.

The particle beam device according to the system described herein has,in addition to the above-mentioned advantages, the advantage that theback focal plane and thus the diffraction plane of the objective isimaged enlarged in the intermediate diffraction pattern plane.Accordingly, the phase-shifting element may be designed similarlyenlarged, so that the phase-shifting element is more easilymanufacturable due to the not very strict dimensional requirements. Itis furthermore advantageous that the second diffraction lens is situatedin or in the area of the intermediate diffraction pattern plane, so thatthe second diffraction lens has an influence, in principle, virtuallyonly on the position of an image plane conjugated with respect to anobject plane of the objective.

The enlarged image of the diffraction plane is basically linearlydistorted due to the at least one multipole element. This is also knownas an anamorphotic image. The enlarged image of the diffraction plane isof a different size in the directions of two mutually perpendicularaxes. The ratio of the enlargement with respect to these two axes isknown as aspect ratio. The aspect ratio influences and defines thedimensions of the phase-shifting element. For the system describedherein, an aspect ratio in the range of 1:10 to 1:100 may beadvantageous.

In one embodiment of the particle beam device according to the systemdescribed herein, the particle beam device has an optical axis. The atleast one multipole element, which is used for the above-mentionedanamorphotic imaging of the diffraction plane, is situated, viewed inthe direction of the second diffraction lens, away from the objective. Afirst multipole element and a second multipole element are possiblyprovided, the first diffraction lens, the first multipole element, thesecond multipole element, and the intermediate diffraction pattern planebeing situated in the direction of the second diffraction lens away fromthe objective. A good anamorphotic imaging of the diffraction plane isensured due to the above-described exemplary embodiment.

In a further embodiment, a third multipole element, a fourth multipoleelement, and a projective lens are also situated on the particle beamdevice according to the system described herein, the third multipoleelement, the fourth multipole element, and the projective lens beingsituated in the direction of the projective lens away from theintermediate diffraction pattern plane. The particle beam device mayalso have a fifth multipole element in the area of the intermediatediffraction pattern plane, which is advantageously split into a firstsubmultipole element and a second submultipole element. Theabove-mentioned arrangement of multipole elements ensures that theanamorphotic imaging is compensated again, i.e., is canceled.Furthermore, the split into two submultipole elements is advantageous inparticular during assembly because the phase-shifting element must besituated in the area of the intermediate diffraction pattern plane andthe split ensures better access.

In an embodiment of the system described herein, the multipole elementor at least one of the multipole elements is provided with at least onemagnetic or electric pole. Depending on the number of poles, desiredfields that are required for anamorphotic imaging may be generated. Themultipole element or at least one of the multipole elements may have aquadrupole field. In particular it is provided that the multipoleelement or at least one of the multipole elements is designed as aquadrupole element.

It is furthermore advantageous to superimpose a dipole field on eachquadrupole element, so that biaxial double deflection systems are formedby two adjacent quadrupole elements.

In a further exemplary embodiment, the particle beam device has at leastsix quadrupole elements, which are excitable antisymmetrically to theintermediate diffraction pattern plane, an octupole being superimposedon each quadrupole element and on the intermediate diffraction patternplane. An arrangement of this type of quadrupole elements and octupolesis known from DE 42 04 512 A1, but not in the context of aphase-shifting element. This exemplary embodiment ensures, on the onehand, anamorphotic imaging of the diffraction plane, the phase-shiftingelement being situated in the image, and, on the other hand, correctionof opening errors and chromatic errors in the particle beam device.

In a further exemplary embodiment of the particle beam device accordingto the system described herein, the particle beam device has at leastone quadrupole septuplet containing six quadrupole elements, thequadrupole elements being excitable symmetrically to the intermediatediffraction pattern plane, and at least five octupoles excitable withinthe quadrupole septuplet. Such an arrangement is known from DE 101 59454 A1, but not in the context of a phase-shifting element. Thisexemplary embodiment again ensures; on the one hand, anamorphoticimaging of the diffraction plane and, on the other hand, correction ofopening errors and chromatic errors of the particle beam device.

As mentioned previously, in an embodiment, the quadrupole element or atleast one of the quadrupole elements has an electrical or magneticdesign. It is also possibly provided that at least one of the octupoleshas an electrical and/or magnetic design.

One embodiment of the particle beam device also provides that theparticle beam device is provided with a corrector for correction ofaberrations, which may be designed as described above. It may beprovided that the multipole element (in particular the above-describedsystem of quadrupole elements) may be situated upstream and/ordownstream from the corrector with respect to a beam direction of theparticle beam device. This exemplary embodiment ensures that in the caseof imaging in the Gauss focus, a zero-position-free contrasttransmission is implemented. The absolute value of the phase contrasttransfer function varies between the absolute value 1 (i.e., optimumphase contrast at 90° phase shift) and 0.707 (45°).

The particle beam device according to the system described herein may bedesigned as an electron beam device, in particular as a TEM.

The system described herein also relates to a particle beam devicehaving at least one objective which has a back focal plane, and at leastone first diffraction lens for imaging the back focal plane of theobjective in an intermediate diffraction pattern plane. Furthermore, atleast one second diffraction lens is provided, which is situated in orin the area of the intermediate diffraction pattern plane. At least onephase-shifting element and at least one multipole element designed as aquadrupole element are also situated on the particle beam device. Thephase-shifting element may have one of the above-mentioned features orfeature combinations, but is not limited thereto. Also in the case ofthis particle beam device, the diffraction plane is imaged, so that thephase-shifting element may be designed appropriately enlargedgeometrically. This ensures, on the one hand, easy manufacturing of thephase-shifting element and, on the other hand, reduction of suppressionof particles of a particle beam by components (for example, holdingframes) of the phase-shifting element as known from the related art.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the system described herein are described belowin greater detail with reference to the following figures:

FIG. 1 shows a top view onto a phase-shifting element in the form of aphase plate;

FIG. 2 shows a sectioned view of the first phase-shifting element alongline A-A in FIG. 1;

FIG. 3 shows a sectioned view of the first phase-shifting element alongline B-B in FIG. 1;

FIG. 4 shows a sectioned view of a second phase-shifting element and atop view according to FIG. 1, the section being along line B-B;

FIGS. 5 a,b show schematic illustrations of distributions of anon-homogeneous potential for phase shifting;

FIG. 6 shows a schematic illustration of a control unit connected to thephase-shifting element;

FIG. 7 shows a schematic illustration of a segmented electrode;

FIG. 8 shows a schematic illustration of a comb-like arrangement ofelectrodes;

FIG. 9 shows a top view onto another phase-shifting element;

FIG. 10 shows a top view onto another phase-shifting element having across-shaped opening;

FIG. 11 shows a schematic illustration of a system of two phase-shiftingelements;

FIGS. 12 a,b show schematic illustrations of two phase-shifting elementshaving cross-shaped openings;

FIG. 13 shows a schematic illustration of another phase-shifting elementin a drift unit design;

FIG. 14 shows a schematic illustration of a top part of a TEM;

FIGS. 15 a,b show schematic illustrations of a bottom part of a TEMhaving a phase-shifting element;

FIGS. 16 a,b show schematic illustrations of a bottom part of a TEMmodified with respect to FIGS. 15 a, b having a phase-shifting element;

FIG. 17 shows a schematic illustration of a beam path in a bottom partof a TEM having multipole elements;

FIG. 18 shows a schematic illustration of a beam path in a bottom partof another TEM having multipole elements;

FIG. 19 shows a schematic illustration of a bottom part of a TEM havingtwo phase-shifting elements; and

FIG. 20 shows a schematic illustration of a phase-shifting elementhaving carbon films.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

The system described herein is based on the recognition that a desiredphase shift is achievable using simple geometric designs with the helpof a non-homogeneous and/or anisotropic potential, in particular anon-homogeneous electric potential. FIG. 1 shows a top view of a firstexemplary embodiment of a phase-shifting element 1 in the form of aphase plate in which a non-homogeneous potential or an anisotropicpotential is generated. Phase-shifting element 1 is situated in the beampath of a particle beam device (for example, an electron beam device inthe form of a TEM) in such a way that the optical axis of the particlebeam device runs in the z direction, which is perpendicular to the planeof the page on which FIG. 1 is printed.

Phase-shifting element 1 has a first dimension E1 in the y direction anda second dimension E2 in the x direction. In principle, the phase platehas a rectangular design in this exemplary embodiment. It should beexpressly pointed out that the system described herein is not limited toa rectangular design of the phase plate. Rather, the phase plate mayhave any conceivable shape. For example, it may also have a rounddesign.

Phase-shifting element 1 is provided with a rectangular through openingin the form of a slot 2 approximately in the center. Slot 2 has a firstdimension O1 in the y direction and a second dimension O2 in the xdirection. Dimension O1 is approximately 60 μm in the exemplaryembodiment illustrated here. Dimension O2 is approximately 2 μm to 3 μm.

FIG. 2 shows a schematic illustration of a sectioned view ofphase-shifting element 1 along line A-A in FIG. 1. Phase-shiftingelement 1, having a multilayer design, has a third dimension E3 in the zdirection, along which slot 2 fully extends. Phase-shifting element 1furthermore has a central electrode 3, which is made of gold and whichis provided with a first side 3 a and a second side 3 b. An insulatorlayer 6 is situated on first side 3 a. Furthermore, an insulator layer 7is situated on second side 3 b. Both insulator layer 6 and insulatorlayer 7 are made of Al₂O₃. Insulator layers 6 and 7 are in turn providedwith a shield layer 4 and 5, respectively. Both shield layer 4 andshield layer 5 are made of gold and are at the potential of thesurroundings. They are grounded, for example.

An axis, which in FIG. 1 runs in the z direction, is provided with thereference numeral 8 in FIG. 2. The axis runs through slot 2. Centralelectrode 3 is controllable with the aid of an electronic unit (controlunit, not illustrated) in such a way that a non-homogeneous electricpotential is obtained in slot 2. The scattered and unscattered particlebeams (for example, electron beams in a TEM) pass through slot 2, theunscattered particle beam (i.e., the particle beam of zeroth order ofdiffraction) suffering a relative phase shift with respect to theparticle beams of higher order of diffraction. The interference of theparticle beams of higher order of diffraction with the relativelyphase-shifted particle beam provides a sufficiently good, contrast-richimage.

FIG. 3 shows a section along line B-B in FIG. 1. Central electrode 3again has two individually controllable electrodes 9 and 10, which aresurrounded by an insulator layer 6 and an insulator layer 7.Furthermore, both electrodes 9 and 10 are isolated from each other by aninsulator layer 11. Also in this exemplary embodiment, insulator layers6, 7, and 11 are made of Al₂O₃. Insulator layers 6 and 7 are in turnprovided with shield layers 4 and 5, respectively, which are made ofgold and are at the potential of the surroundings (for example,grounded).

A first potential (−U_(B)) is generated on electrode 9 via an electronicunit (control unit, not illustrated), while a second potential (U_(B))is generated on electrode 10. The variation of the potential is shown inFIG. 5 b. It is apparent that an undiffracted particle beam passingthrough slot 2 suffers a phase shift of 180° (i.e., π) with respect tothe diffracted particle beams of higher order of diffraction passingthrough slot 2. The exemplary embodiment shown in FIG. 3 is used for theHilbert phase shift and is therefore designed as a Hilbert phase plate.

FIG. 4 shows another exemplary embodiment of a phase-shifting element 1in a multi-layer design according to the system described herein in theform of a phase plate. The same reference numerals are used for the sameelements as in the previously described exemplary embodiments. In topview, this phase-shifting element 1 has the same design as the exemplaryembodiment illustrated in FIGS. 1 and 2. Accordingly, it also has slot2. However, considering a sectional view along line B-B of the furtherexemplary embodiment, it is apparent that it has a somewhat differentdesign from that of the exemplary embodiment of FIG. 3. In the exemplaryembodiment of FIG. 4, electrode 3 is formed by three electrodes 12, 13,and 14 made of gold, which are separated from each other with the aid ofinsulator layers 15 and 16 made of Al₂O₃. As an alternative, theinsulator layers may be formed by vacuum. Additional insulator layers 6and 7 made of Al₂O₃ are situated on the sides of electrodes 12 through14, which are also provided with shield layers 4 and 5 made of gold andbeing at the potential of the surroundings (for example, grounded).

Potentials are again generated via an electronic unit (control unit, notillustrated) in such a way that a potential U_(B) is applied toelectrodes 12 and 14, while a potential U₀ is applied to electrode 13.The variation of the potential is shown in FIG. 5 a. It is apparent thatan undiffracted particle beam passing through slot 2 suffers a phaseshift of 90° (i.e., π/2) with respect to the diffracted particle beamsof higher order of diffraction passing through slot 2. The exemplaryembodiment shown in FIG. 4 is used for the Zernike phase shift and istherefore designed as a Zernike phase plate.

FIG. 6 shows the system of phase-shifting element 1 of FIGS. 1, 2, 3,and 4, and a control unit 103, which is connected to phase-shiftingelement 1 via lines 101 and 102. The above-mentioned electrodes areindividually controllable with the aid of control unit 103 forgenerating the desired potential.

FIGS. 7 and 8 show further embodiments of electrodes which may beimplemented on phase-shifting element 1 according to the systemdescribed herein. FIG. 7 shows a central electrode 13 and two segmentedelectrodes 12 and 14 spaced therefrom by insulators. Electrode 12 thushas segments 12 a through 12 h insulated from each other, whileelectrode 14 has segments 14 a through 14 h insulated from each other.Each individual segment is individually controllable. They are basicallyeach connected to control unit 103, as in FIG. 6.

FIG. 8 shows another embodiment of the electrodes. In this embodiment,electrodes 12 and 14 have a comb-like structure.

FIG. 9 shows another exemplary embodiment of phase-shifting element 1according to the system described herein having a circular opening 17.

FIG. 10 shows another exemplary embodiment of a phase-shifting element1. It is provided with two rectangular slots 18 and 19, which are at aright angle to each other. The two slots 18 and 19 thus form a cross,through whose center 20 the undiffracted particle beam passes. Themultilayer construction of this phase-shifting element 1 is in principlethe same as the construction according to FIG. 3 or 4. This specificembodiment also ensures that a non-homogeneous electric potential isprovided, so that the undiffracted particle beam receives a relativephase shift impressed on it with respect to the particle beams of higherorder of diffraction. A cross phase contrast is produced due to thecross-shaped design.

FIG. 12 a shows another illustration of a phase plate 2 having twocross-shaped slots 18 and 19. Only electrode 3 (central electrode),which is at positive potential, is illustrated here. The 4 internalquadrants are grounded, or a slightly negative voltage is applied tothem for generating a zero potential on the optical axis. A potentialfor a relative phase shift of π/2 is generated in slots 18 and 19.

FIG. 12 b shows another illustration of a phase plate havingcross-shaped slots 18 and 19. In this exemplary embodiment, electrodes104 and 105 are at a positive potential, while electrodes 106 and 107are at a negative potential. A potential for a relative phase shift isagain generated in slots 18 and 19. The potential in the X sectionproduces a relative phase shift of −π/2, while the potential in the Ysection produces a relative phase shift of π/2. Since in the X sectionthe potential is negative in the distant wave zone and in the Y sectionit is positive in the distant wave zone, a zero crossing of thepotential on the optical axis is achieved.

A good non-homogeneous electric potential is generated and asufficiently good phase shift is ensured also by an embodiment which isschematically illustrated in FIG. 11. In this exemplary embodiment, twophase-shifting elements 1 in the form of phase plates are used, each ofwhich has a slot-shaped opening 18 and 19, respectively. The twophase-shifting elements 1 are connected in series in a particle beamdevice (for example, a TEM), a quadrupole optical system 21 beingsituated between the two phase-shifting elements 1. The twophase-shifting elements 1 are situated with respect to each other insuch a way that the two slot-shaped openings 18 and 19 are perpendicularto each other. The two phase-shifting elements 1 provide anon-homogeneous electric potential in such a way that after a particlebeam passes through openings 18 and 19, the undiffracted portion of theparticle beam suffers a phase shift with respect to the diffractedportion of the particle beam, a crossed phase contrast again beingachieved through the serial connection of the two phase-shiftingelements 1.

FIG. 13 shows another exemplary embodiment of phase-shifting element 1in the form of a phase plate. FIG. 13 is a schematic view of one-half ofphase-shifting element 1, which is again provided with slot-shapedopenings 18 and 19, which are arranged in a cross shape. An electrode issituated in opening 18, which forms a drift unit 22 with an oppositeelectrode (not illustrated), which is surrounded by an insulator layerof vacuum or Al₂O₃. Drift unit 22 is designed for providing anon-homogeneous electric potential and therefore also causes thediffracted particle beam to suffer a relative phase shift with respectto the undiffracted particle beam.

FIG. 14 shows the top part of a TEM whose bottom part will be describedin greater detail below. The TEM has an electron source 23 in the formof a thermal field emission source. However, another electron sourcemay, of course, also be used. An extraction electrode 24, whosepotential extracts electrons from electron source 23, is situated alongoptical axis 32 of the TEM downstream from electron source 23.Furthermore, an electrode 25 for focusing the source position and atleast one further electrode 26 for acceleration are provided. Due toelectrode 26, the electrons exiting from electron source 23 areaccelerated with the aid of an electrode voltage to a desired andsettable energy. A design of this type has long been known and will notbe discussed further in detail.

A multistage condenser having three magnetic lenses 27 through 29,followed by an objective 30, is situated further along optical axis 32.An object plane 31, on which a sample to be studied may be placed withthe aid of a sample manipulator, is situated on objective 30. Theelectron source is focused onto the back focus plane of objective 30.

FIGS. 15 a and 15 b show the bottom part of the TEM, which has a systemfor anamorphotic imaging of a diffraction plane using a multipolesystem. FIG. 15 a shows the X section (i.e., the section along the Xaxis), and FIG. 15 b shows the Y section (i.e., the section along the Yaxis). Objective 30 according to FIG. 14 is followed by a firstdiffraction lens 34 along optical axis 32 of the TEM. It is followed bya first quadrupole element 36 and a second quadrupole element 37. Foradjustment purposes, quadrupole element 36 is associated with adeflector system 38, while quadrupole element 37 is associated with adeflector system 39. The two deflector systems 38 and 39 have a dipolefield. A second diffraction lens 41 is situated in intermediatediffraction pattern plane 40, the main plane of second diffraction lens41 corresponding to intermediate diffraction pattern plane 40.Phase-shifting element 1, which in the exemplary embodiment illustratedhere is one of the above-mentioned phase-shifting elements 1, is alsosituated in the main plane of second diffraction lens 41. For example,it is phase-shifting element 1 according to FIGS. 1, 2, and 3. Aquadrupole element 43 is also situated in the area of the main plane ofsecond diffraction lens 41. Further quadrupole elements are providedfurther along optical axis 32. Thus, second diffraction lens 41 isfollowed by a quadrupole element 46 and a quadrupole element 47. Eachquadrupole element 46 and 47 is associated with a deflector system 44and 45, each of which in turn has a dipole field. Quadrupole element 47is followed by a projective lens 48 along optical axis 32.

The operation of the TEM according to FIGS. 14, 15 a, and 15 b isdescribed in greater detail below. Anamorphotic images such as producedby the TEM illustrated here are produced by multipole systems which maybe described by 4 fundamental paths. These are paths x_(α), y_(β), x_(γ)and y_(δ). The fundamental path which begins in the plane of the image(i.e., object plane 31) on optical axis 32 in the X section (imagingbeams) is labeled x_(α). The corresponding Y section is labeled y_(β).The fundamental path which begins (field beams) under a certain angle indiffraction plane 33, which corresponds to the back focal plane ofobjective 30, is labeled x_(γ) in the X section and y_(δ) in the Ysection. The paths satisfy the equations of the set:x′ _(α) ·x _(γ) −x _(α) ·x′ _(γ)=1y′ _(β) ·y _(δ) −y _(β) ·y′ _(δ)=1where the apostrophized quantities stand for the derivatives (slopes) ofthe paths. The set of equations has long been known, so that it is notdiscussed in greater detail here.

The object to be studied is in the focal point of objective 30, so thata parallel bundle of imaging beams exits objective 30. The bundle ofimaging beams is focused in first intermediate image 35 with the aid offirst diffraction lens 34. The front (object-side) focal plane of firstdiffraction lens 34 is in the back focal plane, i.e., diffraction plane33 of objective 30, so that the field beams exit first diffraction lens34 as parallel beams. Quadrupole element 36 is situated in the area ofintermediate image 35 in such a way that it does not influence theimaging beams. However, the field beams are defocused (x_(γ)) or focused(y_(δ)) by first quadrupole element 36. A focused, stigmatic image ofdiffraction plane 33 is obtained in connection with the other quadrupoleelement 37. The imaging beams are not influenced by quadrupole element36, but they are influenced by quadrupole element 37; they are defocusedin one direction (namely y_(β)) and focused in the directionperpendicular thereto (x_(α)). In this way, a diffraction pattern ofdiffraction plane 33 having different enlargements in the two imagesections X and Y is obtained in intermediate diffraction pattern plane40 (which is a plane of symmetry). An anamorphotic (i.e., linearlydistorted) diffraction pattern is thus obtained. With the condition fora focused diffraction patternx _(γ)(ZS)=y _(δ)(ZS)=0,where ZS corresponds to intermediate diffraction pattern plane 40, thepaths and slopes of the paths must satisfy the following equations:x _(α)(ZS)·x′ _(γ)(ZS)=−1y _(β)(ZS)·y′ _(δ)(ZS)=−1

The ratio of y_(β) and x_(α) in intermediate diffraction pattern plane40 (ZS) is the aspect ratio, which in the exemplary embodimentillustrated here is in the range of 1:10 to 1:100. The aspect ratiodefines the dimensions of phase-shifting element 1, which is designed asa rectangle having slot 2. The dimension of slot 2 is selected in such away that at least part of, or even all, the diffraction pattern of backfocal plane 33 passes through slot 2. In other words, the unscattered orundiffracted particle beam of zeroth order, together with at least somescattered or diffracted particle beams of higher orders of diffraction,passes through slot 2.

The use of the combination of phase-shifting element 1 with anamorphoticimaging has the advantage that interfering holding devices are no longerneeded for phase-shifting element 1. Thus, contrary to the related art,in the case of the system described herein, no portion of a particlebeam is suppressed due to shadowing. A good signal and therefore goodimage information is thus obtained. In addition, the use of anon-homogeneous potential ensures that the phase contrast obtainedremains spatial frequency-independent.

Further down the TEM beam path, the anamorphotic image is compensatedagain. Quadrupole elements 43, 46, and 47 are used for this purpose.Second diffraction lens 41 and quadrupole elements 43, 44, and 45influence imaging beams x_(α) and y_(β) in such a way that they runmirror-symmetrically to the anamorphotic diffraction plane, but thefield beams, which are focused in the plane of symmetry, pass throughthe plane of symmetry unaffected. In contrast, field beams x_(γ) andy_(δ) run antisymmetrically to the plane of phase-shifting element 1.Due to the existing symmetry conditions, quadrupole elements 46 and 47situated below intermediate diffraction pattern plane 40, on which againa dipole field is superimposed by deflector systems 44 and 45,compensate the anamorphism of the beam path.

Instead of phase-shifting element 1, which has a design according toFIGS. 1, 2, and 3, a phase-shifting element 1 having a cross-shapedopening (for example, FIG. 10) may also be situated in intermediatediffraction pattern plane 40. Using a suitable connection of thequadrupole elements, the anamorphotic image of diffraction plane 33 maybe rotated by 90°. Thus even the diffracted particle beams, which runperpendicular to the gap, may subsequently be phase shifted.

FIGS. 16 a and 16 b show a bottom part, which is almost identical toFIGS. 15 a and 15 b, of a TEM, which, however, is provided with asubmultipole element in the form of a quadrupole element 43 a and asubmultipole element in the form of a quadrupole element 43 b in thearea of intermediate diffraction pattern plane 40. The split ofquadrupole element 43 shown in FIGS. 15 a and 15 b into two quadrupoleelements 43 a and 43 b is advantageous from the point of view ofassembly, because it facilitates placing phase-shifting element 1 in thebeam path of the TEM using manipulators. The function of quadrupoleelements 43 a and 43 b is elucidated below.

Quadrupole element 43 a causes imaging beam y_(β) to be slightlyfocused, and imaging beam x_(α) to be slightly defocused, so that theyhave a shared virtual imaging plane which is situated in intermediateimage 35. This imaging plane is formed by second diffraction lens 41 inintermediate image plane 49. Quadrupole element 43 b thus operatessimilarly to quadrupole element 43 a and causes the imaging beams to befocused or defocused.

FIG. 17 shows another exemplary embodiment of an imaging system in aTEM, which basically combines three things, namely a phase-shiftingelement, an anamorphotic image of a diffraction plane, and a color anderror correction.

In this exemplary embodiment, the objective, i.e., the objective plane,is labeled with reference numeral 50 in FIG. 17, while the back focalplane, i.e., the diffraction plane, of objective 50 is labeled withreference numeral 51. Furthermore, a number of multipole elements in theform of quadrupoles and octupoles are situated in the beam path alongthe optical axis of the TEM oriented in the z direction. FIG. 17basically shows the positions and intensities of the quadrupolessituated along optical axis z, i.e., of their quadrupole fields Q1through Q7. Quadrupoles Q1 through Q7 are situated symmetrically tocentral plane of symmetry ZS, which is, at the same time, theintermediate diffraction pattern plane and in which a phase-shiftingelement 54 is situated. Phase-shifting element 54 has a structureaccording to FIGS. 1, 2, and 3.

Plane of symmetry ZS is a plane of symmetry with respect to both thequadrupole fields and the fundamental paths. Quadrupoles Q1 through Q7have an electric-magnetic design. They are made of superposed mutualelectrostatic and magnetic quadrupole fields. Image paths 52, i.e., thepaths starting in object plane 50 at a certain angle and labeled x_(α)and y_(β), run symmetrically to plane of symmetry ZS. Field paths 53,i.e., the paths which start at a certain angle in diffraction plane 51and labeled x_(γ) and y_(δ), run antisymmetrically to plane of symmetryZS. Back diffraction plane 51 is anamorphotically imaged in plane ofsymmetry ZS, i.e., imaged linearly distorted, as has been basicallydescribed for the exemplary embodiments of FIGS. 15 a,b and 16 a,b.

The exemplary embodiment of FIG. 17 also has five octupoles O1 throughO5. Octupoles O1 through O5 are indicated with appropriate arrows. Thepositions of the individual octupoles O1 through O5 along optical axis zare selected with respect to the fundamental paths in such a way thatcomponents of the opening error of the objective may be corrected. Oneoctupole pair is situated at points at which the beam is largelyrotation-symmetric such as in areas upstream from first quadrupole Q1and downstream from last quadrupole Q7. Further octupole pairs O2, O3,and O4 are situated in areas of astigmatic intermediate images, i.e., atthe zero crossings of the fundamental paths, because this ensures thatthe error correction is largely uncoupled. Instead of an octupole O3 inplane of symmetry ZS, also two octupoles O6 and O7 may be superimposedon the third and fifth octupole, respectively.

In the exemplary embodiment described in FIG. 17, three essentialaspects are advantageously combined, namely the use of a phase-shiftingelement 54, the anamorphotic imaging of a diffraction plane, and thecolor and error correction with the aid of a multipole system. Thereforeit has, in addition to the above-mentioned advantages and modes ofoperation, also the color and error correction. These may, however, alsobe omitted so that the above-described multipole system is usedexclusively for the anamorphotic imaging, the aspect ratio being easilysettable in this exemplary embodiment.

Another exemplary embodiment of a TEM which has a phase-shiftingelement, an anamorphotic imaging of the diffraction plane of the TEMobjective, and a color and error correction is schematically illustratedin FIG. 18. The optical axis of the TEM is again labeled z. The twomutually perpendicular sections in the x and y directions are projectedperpendicularly to each other.

x_(α) and y_(β) denote the x component and the y component,respectively, of axial beam 55 originating from the axis of an objectpoint (the other fundamental paths are again labeled x_(γ) and y_(δ)).The two components are deflected differently in a first quadrupole 56.An astigmatic intermediate image is obtained, the path running in the Xsection becoming equal to zero and thus intersecting optical axis z.Another quadrupole 57, which due to its position affects only they_(β)-component and leaves the x_(α) component unaffected, is situatedat this point. Another quadrupole 58 is located closest to the pointwhere the y_(β) component intersects optical axis z. At this point thex_(α) component is then influenced in the form of a reversal.

Octupole 60, which is situated symmetrically to plane of symmetry 59, isfurthermore provided. In addition, additional octupoles are providedwhich are not illustrated in this figure.

While the illustrated exemplary embodiment has a symmetric design withrespect to the arrangement of multipoles (plane of symmetry 59),quadrupoles 61 through 63 have opposite polarity with respect toquadrupoles 56 through 58. Although the beam path is asymmetric, arectangular phase plate may also be situated here.

As mentioned previously, out-of-axis beam 64 also has two components,namely the x_(γ) component and the y_(δ) component. These are also splitby first quadrupole 56 and deflected by quadrupole 57.

In this exemplary embodiment a phase-shifting element 54 according toFIGS. 1, 2, and 3 is also provided. It may be situated at two differentpoints. In this case also, the back focal plane, i.e., the diffractionplane, of the objective may be anamorphotically imaged in plane ofsymmetry 59, so that the same advantages as described previously areachieved. Situating the phase-shifting element 54 in the area ofquadrupole 61 is advantageous in particular. A rather large aspect ratioresults here. Although the x_(γ) component runs far away from axis z(perpendicularly to a phase-shifting direction), using a phase-shiftingelement 54 having a cross-shaped opening, the strongly astigmaticundiffracted particle beam (zero beam) passes through without sufferinga phase shift. The diffracted particle beams of higher order, however,suffer a phase shift.

FIG. 19 schematically shows another exemplary embodiment of a bottompart of a TEM. Objective 65, which has a back focal plane, i.e.,diffraction plane 66, is here situated along an optical axis 72 of theTEM. In the back focal plane, i.e., diffraction plane 66, a firstphase-shifting element 73 is also situated, which is designed accordingto FIGS. 1, 2, and 3. A first diffraction lens 67 which is followed by aquadrupole element 68 in the beam path of the TEM is situated further inthe beam path of the TEM along optical axis 72. First diffraction lens67 has an image plane which is, on the one hand, the intermediatediffraction pattern plane and, on the other hand, the conjugated planeto diffraction plane 66 of objective 65. The main plane of a seconddiffraction lens 69 is situated in the intermediate diffraction patternplane. A second phase-shifting element 74, which is also designedaccording to the exemplary embodiment of FIGS. 1, 2, and 3, is alsosituated in this intermediate diffraction pattern plane. Anotherquadrupole element, followed by a projective lens 71, is situateddownstream from second diffraction lens 69. Also in this exemplaryembodiment, an enlarged image of back focal plane 66 of objective 65 inintermediate diffraction pattern plane 69 is provided with the aid offirst diffraction lens 67, so that the previously mentioned advantagesare also achieved here.

It should be expressly pointed out here that the embodiments describedin FIGS. 14 through 19 are not limited to the application of theexemplary embodiment of the phase-shifting element illustrated in FIGS.1, 2, and 3. Rather, any phase-shifting element using which anon-homogeneous or anisotropic potential may be generated for phaseshifting may be used.

FIG. 20 shows a combination of phase-shifting element 1 according to thesystem described herein having two lateral carbon films 104 and 105.This embodiment combines the related art with phase-shifting element 1according to the system described herein.

The system described herein is described with reference to a TEM, i.e.,a transmission electron microscope. It should be expressly pointed out,however, that the present invention is not limited to transmissionelectron microscopes. Rather, the system described herein is applicablein any particle beam device in which the use of a phase-shifting elementis desired, for example, also a transmission ion microscope.

Other embodiments of the invention will be apparent to those skilled inthe art from a consideration of the specification or practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with the true scope and spiritof the invention being indicated by the following claims.

What is claimed is:
 1. A phase-shifting element, comprising: at leastone potential-generating device that generates at least one of: anon-homogeneous potential and an anisotropic potential, wherein, in use,the phase shifting element shifts a phase of at least a portion of aarticle beam.
 2. The phase-shifting element as recited in claim 1,wherein the at least one potential-generating device includes a phaseplate in which the at least one of: the non-homogeneous potential andthe anisotropic potential is generated.
 3. The phase-shifting element asrecited in claim 1, wherein a non-rotationally symmetric opening isprovided on the phase-shifting element in a perpendicular direction to aplane of the phase-shifting element for generating the at least one of:the non-homogeneous potential and the anisotropic potential.
 4. Thephase-shifting element as recited in claim 3, wherein the phase-shiftingelement has a plurality of openings.
 5. The phase-shifting element asrecited in claim 3, wherein the opening is slot-shaped.
 6. Thephase-shifting element as recited in claim 3, wherein the opening isrectangular.
 7. The phase-shifting element as recited in claim 3,wherein the opening is cross-shaped.
 8. The phase-shifting element asrecited in claim 1, wherein the potential-generating device forgenerating the least one of: the non-homogeneous potential and theanisotropic potential generates an electric potential.
 9. Thephase-shifting element as recited in claim 1, wherein thepotential-generating device for generating the at least one of: thenon-homogeneous potential and the anisotropic potential generates amagnetic potential.
 10. The phase-shifting element as recited in claim1, wherein the phase-shifting element has exactly one opening.
 11. Thephase-shifting element as recited in claim 10, wherein thephase-shifting element has a first element dimension along a first axisand a second element dimension along a second axis, wherein the firstaxis is not parallel to the second axis, and wherein the opening extendsboth along the first axis and along the second axis.
 12. Thephase-shifting element as recited in claim 11, wherein the opening has afirst opening dimension along the first axis and a second openingdimension along the second axis, the first opening dimension beinggreater than the second opening dimension.
 13. The phase-shiftingelement as recited in claim 11, wherein the first axis is situatedperpendicular to the second axis.
 14. The phase-shifting element asrecited in claim 11, wherein the phase-shifting element has a thirdaxis, which is not parallel either to the first axis or the second axisand along which there extends a third opening dimension.
 15. Thephase-shifting element as recited in claim 1, wherein the phase-shiftingelement has a multilayer design.
 16. The phase-shifting element asrecited in claim 1, wherein the phase-shifting element has at least oneelectrode.
 17. The phase-shifting element as recited in claim 16,wherein the electrode is segmented.
 18. The phase-shifting element asrecited in claim 16, wherein a first electrode and a second electrode,which are individually controllable and to which different potentialsmay be individually applied, are provided in one plane.
 19. Thephase-shifting element as recited in claim 16, wherein the at least oneelectrode has a first side and a second side, and a first insulator issituated on the first side.
 20. The phase-shifting element as recited inclaim 19, wherein a second insulator is situated on the second side. 21.The phase-shifting element as recited in claim 20, wherein a secondshield unit is situated on the second insulator.
 22. The phase-shiftingelement as recited in claim 19, wherein a first shield unit is situatedon the first insulator.
 23. The phase-shifting element as recited inclaim 1, wherein the phase-shifting element has a drift unit.
 24. Thephase-shifting element as recited in claim 1, wherein the phase-shiftingelement has at least one section having an internal potential.